The present invention relates generally to ceramic honeycomb structures or composites, and more particularly to an improved ceramic honeycomb cell construction having curved walls which are capable of accommodating deformation due to thermal or mechanical stresses in planes normal to the longitudinal axis of the cell.
As used hereinafter, ceramic honeycomb structures or composites refers to a structure comprising a plurality of parallel cells or cellular portions which are defined by interconnected and interrelated partitions to form a body of such cells. Usually, the body of the cells is surrounded by a peripheral wall or skin. Typically, all of the cells, except for those which are adjacent to the skin, are of the same geometry or shape, such as triangles, squares, rhombuses, hexagons, or circles. In order to maximize the exposed surface area contained within the body as a whole, the interconnected and interrelated partitions or walls forming the cells are of minimum thickness, for example, from 0.002 to 0.050 inches.
Such ceramic honeycomb structures have found application as substrates or core members for use in catalytic converters or reactors for treating emissions from internal combustion engines. U.S. Pat. No. 3,783,350 to Dwyer et al. discloses a method of coating such honeycomb substrates with a catalyst for reacting with such emissions. Extrusion methods of making monolithic ceramic substrates useful as catalyst core member are described in U.S. Pat. Nos. 3,790,654 to Bagley and 3,846,197 to Wiley. Dies for use in extruding monolithic substrates are provided in the Bagley patent and in U.S. Pat. No. 3,826,603 to Wiley. Ceramic compositions for catalytic convertor substrates are described in U.S. Pat. No. 3,885,977.
In the operation of catalytic converters, the hot exhaust gases flowing through the cells generate severe non-uniform temperature gradients in directions parallel and normal to the axes of the cells. The gradients established in the latter direction cause very high tangential and radial stresses to be exerted on the substrate and have been recognized as the cause of mechanical failure in the form of cracking or fracturing of peripheral regions of the substrate.
In my paper entitled, "Effects of Cell Geometry on Thermal Shock Resistance of Catalytic Monoliths", (Society of Automotive Engineers paper, No. 750171, February, 1975) various measures for improving the capability of a substrate to withstand thermally induced stresses are described. It is noted therein that the thermal shock resistance of the ceramic honeycomb structure is directly proportional to the coefficient of thermal expansion of the material forming the structure and its mechanical strength in the directions of concern and is inversely proportional to its bulk or structural elastic modulus in such directions. Much effort has been directed in the past to devising compositions and manufacturing processes to produce a substrate having a minimal coefficient of expansion and maximum strength. The development work has been successful, insofar as the ceramic substrates used to date have been found to perform satisfactorily under the conditions existing in the present automotive catalytic devices.
However, it is known that the federal pollution requirements concerning automotive exhaust emissions will be more stringent in the future and more particularly that such future requirements will most likely necessitate conversion at higher temperatures in order to remove nitric oxides from the exhaust gases. In other words, it is expected that the future substrates will be subjected to much higher temperatures. These temperatures will induce more severe thermal gradients which are anticipated to be too great for known prior art substrates to withstand without cracking.
Manifestly, it would be very desirable to be able to utilize prior art compositions and manufacturing techniques in making the substrates which will meet the more stringent requirements. Accordingly, it is an object of the present invention to provide a ceramic honeycomb structure having a cell geometry or shape which will enable the structure to withstand, without mechanical failure or cracking, the thermally induced forces expected to be generated at the higher temperatures, while yet utilizing the same or similar compositions and manufacturing processes employed in making present ceramic honeycomb structures.
It is a major object of this invention to provide a ceramic honeycomb structure with a cell geometry which is capable of deforming in a preinduced manner under stress, whereby stress concentrations at or near the corners of the cells will be minimized.
More particularly, it is an object of the present invention to increase the strain tolerance or thermal shock resistance of a ceramic honeycomb structure or composite by providing such structure with a cell geometry characterized by a lower structural modulus in directions generally parallel to the cell walls and normal to the longitudinal axes of the cells. Collaterally, the present invention contemplates a honeycomb structure or composite having more uniform or nearly isotropic structural elastic moduli in directions contained within planes normal to the longitudinal axes of the cells or cellular portions of the structure.
It is a further object of the present invention to provide a honeycomb structure with a cell shape or geometry including curved walls or partitions, wherein the partitions are curved sufficiently to minimize the anisotropic structural modulus characteristics of straight-sided cell geometries, while yet minimizing the stresses at the ends of the walls or partitions induced by the curvatures of the walls.